1. Field of the Invention
The present invention relates to a control device and a control method of an automatic transmission that is mounted in an automobile to control gear change using a motor.
2. Background Art
In the related art, there is an automatic transmission with start clutch as is disclosed, for example, in JP-A-2002-81472. In this related art, an actuator is provided to the start clutch of the dry single plate type, and the clutch engaging force is adjusted by varying a stroke of the clutch using the actuator. In order to ensure safety and comfortable feeling, the start clutch has to control the clutch engaging force with accuracy.
Also, JP-A-2006-6037 discloses a conventional motor control technique. This technique in the related art relates to a method for suppressing influences of a current ripple when detecting a current flowing in the motor. The motor current is detected at specific timing in response to the motor rotation angle.
As has been described, in a driving state where a vehicle is creeping at a low speed or starts slowly, the engaging force of the start clutch has to be adjusted delicately. When the gear is to be shifted in the automatic transmission with clutch, it is necessary to disengage the clutch first and then to engage the clutch after the gear is shifted while preventing the occurrence of a gear change shock. Hence, in the case of a mechanism that uses the motor as an actuator to control a stroke of the clutch and the rotation angle of the motor is proportional to the stroke of the clutch, a torque amount of the motor has to be adjusted with high accuracy in order to adjust the engaging force of the clutch. Because the motor torque is proportional to a current amount of the motor, the control accuracy of the motor current has to be enhanced in order to enhance the control accuracy of the motor torque. Applying such accuracy enhancement to the transmission control using the motor in the related art, however, raises problems as follows.
Such problems will be described in accordance with an operation to engage the clutch from a state where the clutch is completely disengaged. The clutch mechanism described herein is a mechanism in which the rotation angle of the motor is proportional to a stroke of the clutch, and is therefore capable of adjusting an engagement amount using a torque of the motor. In addition, it has a mechanism by which the clutch returns to an open side in a case where no motive power is provided to the clutch from the motor. The clutch is therefore in an open state in such a case, and when the clutch is to be engaged, it is necessary to provide motive power constantly to the clutch from the motor.
Initially, in a state where the clutch is completely disengaged, the motor is not driven and no current is flowing in the motor.
Subsequently, a clutch stroke is varied to engage the clutch. Because the clutch is in a fully open state, a specific driving pattern is provided to the motor driving circuit to rotate the motor. After the specific driving pattern is provided to the motor driving circuit, the motor starts to rotate with some delay time. It is also necessary to manage the motor torque in this interval until the motor starts to rotate. However, in the motor control technique in the related art, a current is detected in sync with the rotation of the motor. It is therefore impossible to detect a current in a state where the motor is stopped or a state where the motor is rotating at an extremely low rotation speed that falls out of the detectable range of the control device. Accordingly, because the motor torque cannot be managed, it is impossible to control the motor toque with high accuracy.
Subsequently, the motor starts to rotate and a clutch stroke starts to vary. At this point in time, because the motor is rotating, it is possible to control the motor torque with high accuracy on the basis of the current detected in response to the rotation angle of the motor.
The clutch stroke varies more rapidly with a further increase of the motor rotation speed. At this point in time, in a case where a current is detected in response to the motor rotation angle in the same manner as above, the number of current detection times and the number of current computation times per unit time by the microcomputer are increased. This raises a problem that larger load is applied on the microcomputer.
When the clutch is to be engaged thereafter, the rotation speed of the motor is reduced gradually by lessening a variance of the clutch stroke gradually to prevent the occurrence of a shock. Once the rotation speed of the motor is reduced, load applied on the microcomputer does not become excessively large when a current is detected in response to the motor rotation angle, and no particular problem arises.
After the clutch is engaged, the clutch stroke no-longer varies, and the clutch engaging force has to be adjusted according to a driving state. Nevertheless, because the motor is not rotating while the clutch is engaged, it is impossible to detect a motor current in response to the rotation angle of the motor. Hence, as was the case described above, the incapability of controlling the motor torque makes it impossible to adjust the clutch engaging force.